#include<iostream>
#include<cmath>
using namespace std;
int main ()
{int n,a[150000];
 cin>>n;
 for(int i=1;i<=n;i++)
   cin>>a[i]; 
 int sum=0;
 
 for(int j=1;j<=n;j+=2)
    sum+=a[j];
 cout<<sum<<endl;
 system("pause");
 return 0;
} 
